Gaseous diffusion

Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through semipermeable membranes. This produces a slight separation between the molecules containing uranium-235 (235U) and uranium-238 (238U). By use of a large cascade of many stages, high separations can be achieved. It was the first process to be developed that was capable of producing enriched uranium in industrially useful quantities.

Gaseous diffusion is based on Graham's law, which states that the rate of effusion of a gas is inversely proportional to the square root of its molecular mass. For example, in a box with a semi-permeable membrane containing a mixture of two gases, the lighter molecules will pass out of the container more rapidly than the heavier molecules. The gas leaving the container is somewhat enriched in the lighter molecules, while the residual gas is somewhat depleted. A single container wherein the enrichment process takes place through gaseous diffusion is called a diffuser.

Uranium hexafluoride

UF6 is the only compound of uranium sufficiently volatile to be used in the gaseous diffusion process. Fortunately, fluorine consists of only a single isotope 19F, so that the 1% difference in molecular weights between 235UF6 and 238UF6 is due only to the difference in weights of the uranium isotopes. For these reasons, UF6 is the only choice as a feedstock for the gaseous diffusion process.[4] UF6, a solid at room temperature, sublimes at 56.5 °C (133 °F) at 1 atmosphere.[5] The triple point is at 64.05 °C and 1.5 bar.[6] Applying Graham's Law to uranium hexafluoride:

Gaseous diffusion plants typically use aggregate barriers (porous membranes) constructed of sintered nickel or aluminum, with a pore size of 10–25 nanometers (this is less than one-tenth the mean free path of the UF6 molecule).[2][4] They may also use film-type barriers, which are made by boring pores through an initially nonporous medium. One way this can be done is by removing one constituent in an alloy, for instance using hydrogen chloride to remove the zinc from silver-zinc (Ag-Zn).

Energy requirements

Because the molecular weights of 235UF6 and 238UF6 are nearly equal, very little separation of the 235U and 238U is effected by a single pass through a barrier, that is, in one diffuser. It is therefore necessary to connect a great many diffusers together in a sequence of stages, using the outputs of the preceding stage as the inputs for the next stage. Such a sequence of stages is called a cascade. In practice, diffusion cascades require thousands of stages, depending on the desired level of enrichment.[4]

All components of a diffusion plant must be maintained at an appropriate temperature and pressure to assure that the UF6 remains in the gaseous phase. The gas must be compressed at each stage to make up for a loss in pressure across the diffuser. This leads to compression heating of the gas, which then must be cooled before entering the diffuser. The requirements for pumping and cooling make diffusion plants enormous consumers of electric power. Because of this, gaseous diffusion is the most expensive method currently used for producing enriched uranium.[10]

The preparation of UF6 feedstock for the K-25 gaseous diffusion plant was the first ever application for commercially produced fluorine, and significant obstacles were encountered in the handling of both fluorine and UF6. For example, before the K-25 gaseous diffusion plant could be built, it was first necessary to develop non-reactive chemical compounds that could be used as coatings, lubricants and gaskets for the surfaces that would come into contact with the UF6 gas (a highly reactive and corrosive substance). Scientists of the Manhattan Project recruited William T. Miller, a professor of organic chemistry at Cornell University, to synthesize and develop such materials, because of his expertise in organofluorine chemistry. Miller and his team developed several novel non-reactive chlorofluorocarbonpolymers that were used in this application.[11]

Calutrons were inefficient and expensive to build and operate. As soon as the engineering obstacles posed by the gaseous diffusion process had been overcome and the gaseous diffusion cascades began operating at Oak Ridge in 1945, all of the calutrons were shut down.[12] The gaseous diffusion technique then became the preferred technique for producing enriched uranium.[2]

At the time of their construction in the early 1940s, the gaseous diffusion plants were some of the largest buildings ever constructed.[citation needed] Large gaseous diffusion plants were constructed by the United States, the Soviet Union (including a plant that is now in Kazakhstan), the United Kingdom, France, China, and South Africa. Most of these have now closed or are expected to close, unable to compete economically with newer enrichment techniques. However some of the technology used in pumps and membranes still remains top secret, and some of the materials that were used remain subject to export controls, as a part of the continuing effort to control nuclear proliferation.